[PATCH v2] drm: Release driver references to handle before making it available again

[Chris Wilson]

When userspace closes a handle, we remove it from the file->object_idr
and then tell the driver to drop its references to that file/handle.
However, as the file/handle is already available again for reuse, it may
be reallocated back to userspace and active on a new object before the
driver has had a chance to drop the old file/handle references.

Whilst calling back into the driver, we have to drop the
file->table_lock spinlock and so to prevent reusing the closed handle we
mark that handle as stale in the idr, perform the callback and then
remove the handle. We set the stale handle to point to the NULL object,
then any idr_find() whilst the driver is removing the handle will return
NULL, just as if the handle is already removed from idr.

v2: Use NULL rather than an ERR_PTR to avoid having to adjust callers.
idr_alloc() tracks existing handles using an internal bitmap, so we are
free to use the NULL object as our stale identifier.

Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>
Cc: dri-devel at lists.freedesktop.org
Cc: David Airlie <airlied at linux.ie>
Cc: Daniel Vetter <daniel.vetter at intel.com>
Cc: Rob Clark <robdclark at gmail.com>
Cc: Ville Syrjälä <ville.syrjala at linux.intel.com>
Cc: Thierry Reding <treding at nvidia.com>
---
 drivers/gpu/drm/drm_gem.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 2e8c77e71e1f..d1909d1a1eb4 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -294,18 +294,21 @@ drm_gem_handle_delete(struct drm_file *filp, u32 handle)
 	spin_lock(&filp->table_lock);
 
 	/* Check if we currently have a reference on the object */
-	obj = idr_find(&filp->object_idr, handle);
-	if (obj == NULL) {
+	obj = idr_replace(&filp->object_idr, NULL, handle);
+	if (IS_ERR(obj)) {
 		spin_unlock(&filp->table_lock);
 		return -EINVAL;
 	}
 	dev = obj->dev;
+	spin_unlock(&filp->table_lock);
 
 	/* Release reference and decrement refcount. */
+	drm_gem_object_release_handle(handle, obj, filp);
+
+	spin_lock(&filp->table_lock);
 	idr_remove(&filp->object_idr, handle);
 	spin_unlock(&filp->table_lock);
 
-	drm_gem_object_release_handle(handle, obj, filp);
 	return 0;
 }
 EXPORT_SYMBOL(drm_gem_handle_delete);
-- 
2.7.0.rc3